Freeze drying
About 3.7 g of fresh apple slices were placed in test tube and
frozen at35C for 48 h, were frozen by immersion in liquid nitrogen immediately before the freeze-drying process and freeze dried
using a Lyovac Gt 2 laboratory freeze dryer. Freeze drying was performed under constant vacuum conditions, to obtain initial sample
temperatures below the glass transition temperature of the tissues
(Tg0=30C) (Karathanos, 1999). The vacuum was controlled by
leaking air through one of the pressure-release valves. Residual
water was removed by incubation over P2O5in vacuum desiccators
at 20C for 12 h. The mass of the samples are measured using the
digital balance before starting the drying procedure, at the end and
after incubation over P2O5. Three replicates are carried out for each
experiment; the time necessary to obtain an invariable weight was
twenty hours.